As devices that read an optical pattern such as a barcode, laser-type devices, line-sensor-type devices using a CCD line sensor, and image-sensor-type devices using a CMOS sensor are practically used. The laser-type devices are of a type in which scanning with a laser light is performed using a vibrating mirror and a change in intensity of reflected light with time is detected, and have a significant advantage in terms of cost since an inexpensive optical sensor can be used.
In recent years, line-sensor-type devices and image-sensor-type devices have an improved reading rate (success rate of reading), and accept wide barcodes. Therefore, laser-type devices are required to have an improved reading rate and accept wide barcodes as well. As an invention of the laser-type device that improves the reading rate, an invention that uses a variable focus mirror has been proposed (see Patent reference 1). In this method, the variable focus mirror is disposed between a laser diode and an optical scanner, and is driven at a speed that is slower than the laser scanning speed. Accordingly, scanning in which a focal length is varied in each scanning step is performed, that is, the laser beam waist position varies periodically, thus enlarging the readable range and improving the reading rate.